Air spring suspensions on railcars have been used for many years. Among the reasons for this is that air springs provide a better vertical ride and are quieter than other types of suspension systems involving mechanical springs and parts.
One of the problems related to air springs is that they may lose air and collapse. Rubber blocks are sometimes installed inside the air bags at some specified distance under the car body to serve to support the car body when the air springs fail. However, the vertical spring rate of the rubber block is normally too high to limit the total deflection of the system, that at high speeds the vibration of the car becomes intolerable. Other types of emergency springs have been used in case of air spring failure, but generally the use of such emergency springs have involved either an uncomfortable ride for the passengers in the car or have required that the car be moved at a relatively low speed.